The generation of three dimensional (3D) objects by imaging a liquid resin is well known and has been commercially available for many years. Typically the liquid resin is made to polymerize in areas exposed to intense UV light from a laser or a mask illuminated by a UV lamp. The two best known applications are building 3D models by a process known as stereolithography and manufacturing flexographic printing plates. Flexographic printing plates are printing plates having considerable surface relief previous methods for creating a 3D object by imaging a liquid resin use a photonic principle, Such processes fall under what is known as the "Law of Reciprocity". This law states that imaging a long time using a low intensity light or imaging a short time using a high intensity light will give the same result, as long as the exposure (defined as the integral of the light intensity over time) stays the same. A different way to state this behaviour is to say that the exposure process falls under the law of linear superposition. The law of linear superposition states that: .function.(a+b)=.function.(a)+.function.(b). Simply stated, the result of the combined exposure of (a+b) is the same as exposure by "a" followed by exposure "b".
Because of this behaviour, it is not possible to focus the exposure deep inside the liquid without exposing the volume above the desired exposure point. This is shown in FIG. 1, which shows the prior art. When beam 3 is focussed by lens 4 to a point 5 inside liquid resin 1, in order to polymerize it, the area above point 5 will undergo polymerization as well. As point 5 moves along a line inside the liquid resin in the immediate vicinity of the line along which point 5 moves the resin is subject to intense exposure for a short time. The volume 6 above the line through which the exposing light passes before reaching point 5 is subjected to a weak exposure for a long time (due to the large overlap of the beams forming point 5). Since the product of intensity and exposure is about the same in volume 6 as it is along line 5, volume 6 will polymerize as well. If the absorbance of material 1 is high, volume 6 will actually receive a higher exposure than the desired area along the line travelled by point 5 as point 5 is scanned to cover a large area. A volume deeper in the fluid 1 than point 5 will be exposed as well, however, with a lower exposure due to part of the light being absorbed. For these reasons, prior art systems can only expose the top layer of a liquid polymer and require elaborate means to lower the polymerized layer and keep it submerged, in order to build an object layer-by-layer, always exposing only the top layer. There are some polymerization processes which deviate from the "Law of Reciprocity" such as two-photon absorption processes, in which the rate is proportional to the square of the intensity. Such processes still integrate light and suffer from very low sensitivity requiring high amounts of UV light.
Recently, a new class of materials known as thermosensitive, or thermal, materials has become available. Their main use is the generation of planographic two dimensional offset printing plates, also known as lithographic plates. Thermosensitive materials polymerize upon reaching a critical threshold temperature and have insignificant polymerization at temperatures below the threshold. Because of this property they operate completely outside the "Law of Reciprocity" or the principle of linear superposition. An example of a thermosensitive process is melting. A block of lead can be melted by heating it up to 500.degree. C. but cannot be melted by heating it up twice to 250.degree. C. It can be kept forever at 250.degree. C. without any effect. This non-integrating behavior is typical of all thermosensitive materials
Some prior art processes use laser heating for stereolithography by cutting thin sheets or melting a thin layer of powder. However, neither process is suitable for true 3D imaging as the material used will scatter the light; it can only be used on a thin layer. Furthermore, in these processes the material starts off as a solid and the heat turns it into a liquid or gas. In this disclosure the word "resin" should be broadly understood as any material capable of being converted from a liquid into a solid by a chemical reaction.